Hydrogen is considered as a promising new energy source. Currently, as there are many technical bottlenecks in its transportation in a high density, chemical solutions are relied on to produce hydrogen in the field for meeting practical needs.
One of such solutions is to inject an aqueous solution of a borohydride, particularly sodium borohydride (NaBH4), as a hydrogen source, added with a small quantity of a strong base as a reagent for preventing too rapid hydrolysis of it, into a fixed bed reactor filled with a catalyst, to cause high-speed generation of hydrogen gas from the reaction NaBH4+(2+n) H2O=NaBO2.nH2O+4H2. Despite its several advantages such as relatively controllable start and stop of hydrogen generation, this method still suffers from many deficiencies, in particular, a low hydrogen production density—4.6 wt % obtained on a stoichiometric basis, with borohydride being adequately reacted at a concentration of 22%. The U.S. company, Millennium Cell Inc., which invented and promoted this method finally went bankrupt.
For this reason, subsequent improvement efforts have been made, focusing primarily on the direct hydrolysis of solid borohydride.
For instance, Chinese Patent Application No. 02121472.7 discloses a vehicle-mounted hydrogen generator which utilizes the hydrolysis of a solid. The solid reactant may be CaH2 or NaBH4 containing a catalyst or a mixture thereof. The catalyst may contain active substances such as Fe, Co, Ni, Cu and Mn. The NaBH4 or the mixture of NaBH4 and CaH2 is added into a solution containing one or more of Fe3+, Fe2+, Co2+, Ni2+, Cu2+ and Mn2+ ions to initiate a hydrolysis reaction. As the drawbacks of this technique have been well discussed and demonstrated in subsequent patent applications in this regard, they will not be further detailed herein.
Additionally, a U.S. company, Rohm and Haas, proposed a “Method for Generation of Hydrogen Gas from Borohydride” (see Chinese Patent Application No. 200710006384.8 or U.S. Patent Application Ser. No. 60/774,258), which discloses a method for generating hydrogen by way of combining a solid composition containing borohydride and one or more bases with an aqueous solution of one or more acids. Compared to other conventional techniques that do not employ a base, this method is advantageously capable of relatively rapidly stopping the generation of hydrogen. However, as the main objective of this reference is the rapid stop of the hydrolysis reaction, it provides no means to detect whether the NaBH4 has been adequately hydrolyzed, nor any means to determine a needed amount of water for adequate hydrolysis of the NaBH4.
Moreover, the U.S. company, Rohm and Haas, further provided a “Method for Generation of Hydrogen Gas” (see U.S. Patent Application Ser. No. 61/201,390 or Chinese Patent Application No. 200910258540.9), which describes a method that is capable of producing hydrogen with reduced foaming and is achieved by combining, in the presence of a transition metal catalyst, water with a solid composition containing borohydride and a base. However, as its main objective is foaming reduction, the reference neither provides any means to detect whether the NaBH4 has been adequately hydrolyzed, nor any means to determine a needed amount of water for adequate hydrolysis of the NaBH4.
In addition, Chinese Patent Application No. 201010261788.3 discloses a method for high-speed, high-safety hydrogen generation from a borohydride-containing composition as a hydrogen source and a portable hydrogen generator. The borohydride-containing composition is a mixture in a ratio of moles of boron atoms of the borohydride to moles of boron atoms of a boron oxide of 1:4-0.5. The hydrogen generator includes an enclosed container that holds the mixture and is provided with, situated under the mixture, a gasbag and/or a mechanical agitator driven by air pressure gradients. This method addresses the shortcomings of the aforementioned hydrogen generation methods either utilizing a hydrolysis of a borohydride solution with a strong base or using a catalyst and is capable of a high hydrogen production rate, hydrogen generation stoppage in 3-10 seconds, high safety and stability under normal temperature and pressure conditions, and long-term storage of the mixture without self-discharge of hydrogen. Furthermore, in the Background section of the reference, it is pointed out that, during the hydrogen generating reaction of a mixture of pulverized NaBH4 and an activating agent (which is implemented as an acid, e.g., H3BO3, in a solid form) with water, as described in U.S. Pat. No. 7,438,732B2, if the solid product resulting from the reaction is not smashed in a timely way, it will agglomerate into large lumps which can block the contact between water and the reactant and thus halt the reaction, and that although the reaction can be resumed by adding a sufficient amount of water to dissolve the lumps therein, this will suddenly create extensive contact between the water and the reactant which will cause the reaction to proceed uncontrollably at a high rate to lead to a risk of explosion.
Further, Chinese Patent Application No. 201010114398.3 discloses the use of mechanically ball-milled lithium borohydride (LiBH4) and porous carbon as a hydrogen source, which is brought into contact with liquid water or water vapor introduced at a controlled flow rate or a saturated pressure, in order for LiBH4 to be hydrolyzed to produce hydrogen gas in a desired amount or at a desired speed. By controlling a weight ratio of lithium borohydride to porous carbon in the range of 93˜90%:7˜10%, compared to those using NaBH4, this solution is capable of a sustained stable rate and a higher efficiency in a more controllable way without involving the use of any catalyst. Further, the reference points out, in its Background section, that due to an enveloping effect of the solid product, a NaBH4 reaction system cannot achieve full release of hydrogen molecules if there is not a suitable catalyst to accelerate the reaction, and in the Summary of Invention section that, the invention is capable of sustained stable hydrogen production at an efficiency higher than that of a technique utilizing the hydrolysis of NaBH4.
Furthermore, Chinese Patent Application No. 201010258192.8 discloses a hydrogen generator, including: a tank for holding a liquid reactant; a porous structure impregnated with a solid reactant; and a guidance means connected to the tank for guiding the liquid reactant into the porous structure to react with the solid reactant to produce hydrogen gas. The porous structure may be prepared by introducing a solution of the solid reactant in the intact form of the porous structure and causing the solid reactant to precipitate in pores of the structure by means of heating. Optionally, prior to the impregnation of the solid reactant, a solution of a catalyst can be first introduced in the intact porous structure and then heated to cause the catalyst to precipitate in pores of the structure. The solid reactant may include sodium borohydride, magnesium hydride, calcium hydride or aluminum. The catalyst solution may include an aqueous solution of cobalt chloride, iron chloride, cobalt sulfate or nickel chloride. The catalyst may include cobalt chloride, iron dichloride, cobalt sulfate or nickel chloride. The liquid reactant may be water, an aqueous solution of malic acid, an aqueous solution of citric acid, an aqueous solution of sulfuric acid, an aqueous solution of sodium bicarbonate, an aqueous solution of calcium carbonate, an aqueous solution of cobalt dichloride, an aqueous solution of cobalt chloride, an aqueous solution of iron dichloride, an aqueous solution of cobalt sulfate, or an aqueous solution of nickel chloride.